Surprising high-energy X-ray emissions from the Whirlpool Galaxy and its small companion

An article published in “The Astrophysical Journal” presents an in-depth spectral analysis of the two active galactic nuclei and other X-ray sources of the two galaxies that form M51. A team of researchers used NASA’s NuSTAR space telescope to detect high-energy X-ray emissions, which can pass through the layers of dust and gas that orbit the two supermassive black holes at the center of the two galaxies that are interacting in an initial phase of a galactic merger. A surprise came from the emissions of a neutron star in the Whirlpool Galaxy, the larger of the pair.

The pair of galaxies called M51 is formed by the dwarf galaxy M51b, also known as NGC 5195, and the spiral galaxy M51a, also known as NGC 5194 or as the Whirlpool Galaxy, which are interacting in an initial phase of a galaxy merger, although the significant difference in size between the two galaxies probably makes it more appropriate to say that NGC 5194 will devour its smaller companion. The process is extremely slow and NGC 5195 has been periodically passing through one of the arms of the biggest companion’s spiral for who knows how long already.

This process has already been the subject of various studies by teams of astronomers, also due to the effects it has on the two supermassive black holes, with the consequence that the two galaxies have as many active galactic nuclei. For example, an article published in the journal “Monthly Notices of the Royal Astronomical Society” in July 2017 described a kind of indigestion for the supermassive black hole at the center of the dwarf galaxy M51b.

The electromagnetic emissions generated by the heating of dust and gas that orbit supermassive black holes are truly remarkable and yet it has happened that the low energy X-rays detected by the M51 ones are lower than expected. The scientists think that the amount of dust and gas around them is such as to block part of the emissions but the NuSTAR space telescope, launched in June 2012, can detect high-energy X-rays that can pass through those layers.

The image (NASA/JPL-Caltech, IPAC) shows the M51 pair at optical frequencies from the Sloan Digital Sky Survey with the Whirlpool Galaxy much larger that its small companion. The high-energy X-ray sources detected by the NuSTAR space telescope are shown in green: the ones at the center of the two galaxies are generated by the heating of dust and gas that orbit the two supermassive black holes, but there are others, such as the neutron star whose emissions are visible on the left edge of the Whirlpool Galaxy.

There’s no comparison between the size of a neutron star, which has a diameter of about a dozen kilometers for a mass that can’t be much more than twice the Sun’s, and those of a supermassive black hole: the one at the center of the dwarf galaxy M51b has a mass estimated around 19 million solar masses. Despite this, neutron stars can emit remarkable amounts of high-energy X-rays, the so-called ultraluminous X-ray sources (ULXs), in this case comparable to those of the two galaxies’ supermassive black holes.

Murray Brightman of Caltech, the lead author of this research, expressed surprise at this discovery since the processes of galaxy merger are supposed to lead to a growth of supermassive black holes and consequently to an increase in their activity and their emissions. The proposed explanation is that this activity isn’t constant but intermittent: on this point, Daniel Stern of NASA’s JPL, a scientist of the NuSTAR mission, commented that the idea was that that activity would change over millions of years but recent studies indicate that such changes could take place in much shorter timescales.

In essence, this study is interesting for different reasons since it includes observations of the activity of the two M51’s supermassive black holes, part of the research on their variability, and those of other ultraluminous X-ray sources that could all be neutron stars. They’re all extreme objects that are studied for various reasons since their activity can have significant effects on the surrounding areas or even on the entire galaxy and allows to perform verification of cosmological models.

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